1. Field of the Invention
This invention relates to a process for the preparation of high-quality pyromellitic dianhydride with a high efficiency by thermal dehydration of pyromellitic acid.
In recent years, due to its unique characteristics, pyromellitic dianhydride is used in a various field as a raw material for super thermal resistant resins. Its typical application is as electronic material.
2. Description of the Prior Art
As to processes for preparing pyromellitic dianhydride by dehydration of pyromellitic acid, there are a process for purifying the product by sublimation concurrently with the dehydration reaction (Japanese Patent Publication No. 5699/1975), a process for effecting the dehydration reaction in a solvent such as acetic anhydride, diphenyl ether, etc., a process for effecting the dehydration in a molten state, and a process for carring out the dehydration in a solid state using a dryer (U.S. Pat. No. 2,937,189).
Among these processes, the sublimation purification process is not satisfactory due to the fact that thermal decomposition is liable to take place and the product is colored since the material is subjected to a relatively high temperature in this process. The process of using acetic anhydride has an advantage in that the material can be homogeneously heated. On the other hand, however, this process has disadvantages, since in the process, recovery of the solvent used is indispensable and it is difficult to eliminate the solvent completely from the product upon completion of the reaction. The process for conducting dehydration in a molten state is not advantageous since it needs a complicate apparatus and it results in deterioration and charring of the product. On the other hand, the process of dehydrating in a solid state is advantageous in that it does not require use of a solvent thus saving procedures for the solvent separation and recovery, and further that it is free from deterioration or decomposition of the compound as long as extremely high temperature is not used.